The present invention relates to a driving device, mainly intended for the wiper system of a motor vehicle, which possesses a housing, an electric motor located in the housing with a pivoted armature shaft, a gear unit located in the housing with a worm shaft located on one section of the armature shaft, and an axial force generating device to compensate for the end play in the armature shaft.
The housing of the driving device is subdivided into a section in which the electric motor is located, and a section in which the gear unit is located. The section of the housing in which the electric motor is located will be described henceforth as the motor housing, the section of the housing in which the gear unit is located will be described as the gear housing.
A driving device of this type is known, for example, from German patent application 196 52 929. The driving device revealed there is used mainly to drive a windshield wiper system in a motor vehicle. It possesses an electric motor which is flange-mounted to a gear housing. The electric motor possesses an armature shaft which extends into the gear housing at its free end. The free end of the armature shaft has a worm shaft to drive a worm gear of a gear unit housed in the gear housing.
The sloping flanks of the worm shaft and the worm gear cause an axial force during operation of the driving device, which impinges on the armature shaft. The direction of the axial force depends on the direction of rotation of the armature shaft. At the points where the windshield wiper changes direction, the direction of the axial force on the drive shaft reverses briefly, because the gear unit is being loaded in the opposite direction. Because of manufacturing tolerances for the individual components of the driving device and because of operating wear in the area of the thrust bearings of the drive shaft, the result can be a relatively large end play of the drive shaft in its thrust bearings. The consequence of the end play can be abrupt axial movement of the armature shaft when the windshield wiper reverses direction, which causes irritating noises.
In order to reduce this end play, the armature shaft in the known driving device runs in two roller bearings, on both sides of the worm drive. The roller bearings have an inner race located on the armature shaft and a fixed outer race attached to the gear housing. The inner race of one of the two roller bearings is located such that it can move on the armature shaft. An axial force generating device bears against the inner race and exerts an axial force on the armature shaft relative to the inner race. In the area of the other roller bearing, a fixed thrust washer is mounted to the armature shaft. The inner race of the other roller bearing is supported against the armature shaft through the thrust washer in such a way that it transmits the axial thrust acting on the armature shaft to the inner race of the other roller bearing. In this way the armature shaft is positioned between the roller bearings in the axial direction. The positioning forces are passed into the gear housing through the roller bearings. This positions the armature shaft in the axial direction relative to the gear housing.
The axial force generating device possesses a spring element which bears against the armature shaft at one end and at the other end against the inner race of one of the roller bearings. The spring element of the axial force generating device has to absorb all of the axial forces from the armature shaft being generated in the direction of the gear unit while the driving device is operating. Moreover, this known driving device requires the use of at least two roller bearings, between which the armature shaft can be kept positioned.
The task of the present invention is to continue the development and design of the driving device of the prior type described above such that it is constructed in the simplest way, that the axial force generating device does not have to absorb all the axial forces from the armature shaft and that it can still fully compensate for the end play of the armature shaft.